As part of the documentary, Sally took park in a flashmob to raise awareness of the condition

And that scene from her new documentary is exactly what Sally wants the world to see because, as the mother of a son with Down’s syndrome, her life is often perceived as challenging.

“People stop you, tilt their head and are sympathetic because they think you’re having a dreadful time,” explains Sally, 46, who is mum to Ollie, 11, who has Down’s syndrome, Luke, eight, and four-year-old Tom.

“You feel like a fraud, because you really aren’t having a terrible time. That realisation made me feel that someone ought to change the public narrative.”

Ollie was diagnosed 10 days after his birth and, while it was a shock at first, Sally was determined it wouldn’t hold him back.

“As an actress, I’ve got a melodramatic side, so I threw myself wholeheartedly into playing a role,” she says. “I became a tiger mother: ‘I will fight them on the beaches! I will get you to the Olympics! I will fight the state if I have to!’”

Sally was spurred into action by proposals to make Non-Invasive Prenatal Testing (NIPT) – a screening that is believed to determine with 99 per cent accuracy whether a woman’s unborn baby has Down’s syndrome – available on the NHS.

She decided to make a documentary about the ethics of screening and the reality of what it’s like to be the parent of a child with Down’s.

“Screening was first introduced in 1986 for the purpose of reducing Down’s because it was cheaper than the lifetime cost of looking after a person with the condition,” explains Sally. “But we’ve changed now and we don’t believe that’s an ethical reason for screening.

“What is clear about NIPT is that it’s a great benefit in that it gives you a choice. However, it has only been bad for the Down’s community.

“In the future, there may be things we can do to improve neurological development of the foetus, but at the moment, the only thing you can do with that knowledge is terminate. And you can terminate a baby with Down’s syndrome up until birth – that doesn’t seem okay to me.”

Sally is also keen that information given to expectant mothers of babies with Down’s syndrome is revised, and questions if a leaflet is the best form of communication.

“In some cases, the statistics are so wildly wrong that they don’t help a woman make a good choice,” she says.

In the emotional one-off documentary, Sally meets Down’s syndrome advocates, doctors, scientists, people with the condition and a woman who chose to terminate after being told she was carrying a Down’s syndrome child.

Sally says she is pro-choice but is keen for the public to see that life with a Down’s syndrome family member can be wonderful

“It’s very important to say that this film and the BBC are not against the test and we’re not against termination. I, myself, am pro-choice,” reveals Sally. “But research in the United States has found that 97 per cent of families with a Down’s syndrome family member are incredibly content.

In fact, 79 per cent are not only content, but feel like their life has been improved by having a loved one with Down’s syndrome.”

That is certainly Sally’s experience.

“The people who I know that have Down’s syndrome are more admirable human beings than people without Down’s,” she says.

“Ollie is very funny, very loving and he treats everybody the same. I imagine he’d show the Queen the same level of respect he shows to Oliver, our postman. Is that bad? I don’t think so.

“I was told it was going to be a tragedy but it feels like it’s a sitcom.

“It’s the kind of sitcom where the police are sometimes called and paint gets thrown, but at the end it’s all right and the characters look back and go: ‘That’s quite funny actually.’ That feels like our family.”